The Weekly Chirp: Fall plumage

Although our calendars proclaim Sept. 21 as the official first day of fall, any New Englander will tell you that it doesn’t really begin until the humid summer days are replaced with crisp afternoons and cool evenings. With this transition in climate arrives perhaps a more conspicuous change: the abrupt emergence of leggings and flannels. Yes, when fall has truly fallen, there is a noticeable shift in fashion. Students around campus replace their light sundresses with dark leggings, or their thin tank tops with sturdy flannels. Some do it to warm their extremities against the chilling fall days, while others simply seek to exploit a new section of their wardrobe. Whatever the motive may be, this pronounced alteration in temperature bestowed on us by Mother Nature is indubitably the cue that drives it.

Some migratory birds respond to this cue as well, both behaviorally and physically. One of North America’s best-studied migratory songbirds, the white-throated sparrow, is a wonderful example of how temperature can affect migratory behavior. When this species decides to migrate south from its breeding grounds in Canada is mainly determined by weather patterns — as the temperature declines and summer turns to autumn, white-throated sparrows realize that it’s time to fly south. Just as cooler days steer us towar our flannels, they steer these charismatic sparrows towards the southern United States.

Fall weather also marks the transition between plumages for many birds, such as in songbirds like warblers. The term "plumage" refers to all of a bird's feathers as a collective unit, and a bird’s plumage can change multiple times over the course of a year — from breeding plumage to fall plumage to winter plumage and back again to breeding plumage. As warblersconclude the breeding season and begin fall migration, the exuberant reds, oranges, yellows and blues of their breeding plumage give way to dull olives and pale browns. However, unlike humans, this is not in response to temperature. The primary interest of warblers shifts from breeding in the summer to migrating in the fall, and the bright colors of their breeding plumages attract undesired attention from hungry hawks and falcons. Replacing those colors with drab pigments allows the warblers to remain secretive and focus exclusively on survival.

While birds in the New England area acquire their dull pigments and prepare for migration, birds in the Southern Hemisphere do the exact opposite. Because the seasons are flipped between hemispheres, birds in places like South Africa and Patagonia have love on their mind — November marks the return of migrants in these areas and therefore the breeding season. Song and sexual angst will saturate the air as males display their alluring colors and females flirt, until that moment when two soulmates encounter one another through the midst of commotion and attempt to produce a new generation.

Alas, in the Northern Hemisphere, one thing is certain: Humans and birds alike, our fall plumages are an exhibition for the world to admire.

Love,

Henry